Another solution is to add a new hard disk to the machine, move the whole /opt/directory to the new partition and create a symbolic link: /opt/bitnami -> /new_hardisk/bitnami. You can also add a new entry in the fstab file to mount the new hard disk automatically.

Hi Beltran, thanks for your help!! The tutorial looks promising, however I failed because the command pvcreate was not found, it seems to be a tutorial for Debian Linux, but the bitnami-VM has Ubuntu installed.
P. s. for your second suggestion: could you tell me which commands to use to correctly move the whole directory tree to the new disk? (My Linux knowledge is very limited)

I know this is a old article but I ran into this yesterday and found a super easy way to do this. I use a program called gpart I go into the VM BIOS and change the boot order to CD first then Hard drive and then put the gpart ISO on the CD and then boot I run the program and allocate the space like you do for windows and save and then shutdown and remove the disk. Reboot and you have the new space.